Wind turbines are commonly used since centuries to convert wind into work. A wind turbine typically is an impulse turbine. The turbine changes the direction of flow of the wind and the resulting impulse spins the turbine and leaves the wind with diminished kinetic energy. If the mechanical energy is used to produce electricity, the device may be called a wind generator or wind charger. If the mechanical energy is used to drive machinery, such as for grinding grain or pumping water, the device is usually called a windmill or wind pump. Developed for over a millennium, today's wind turbines are manufactured in a range of vertical and horizontal axle types. The most common and oldest ones are the horizontal axle turbines, wherein the axles are positioned in line with the direction of the wind (i.e. the direction in which the wind blows). The smallest turbines are used for applications such as battery charging or auxiliary power on sailing boats, while large grid-connected arrays of turbines are becoming an increasingly large source of commercial electric power.
One of the developments of the last decades is to devise wind turbines that are suitable for low wind velocities (typically below 10 m/s, about 5 Beaufort). In particular at lower heights, above land and in the presence of buildings, wind velocity is often too low to economically extract energy from common wind turbines. Recent developments include the Darrieus (including giromill and cycloturbine) and Savonius wind turbines which may generate mechanical energy even at a wind velocity below 5 m/s (about 3 Beaufort). Unlike the Savonius wind turbine, the Darrieus is a lift-type turbine. Rather than collecting the wind in cups (also called “buckets” in the art, referred to as wind receptacles in this specification) dragging the turbine around, a Darrieus uses lift forces generated by the wind hitting aerofoils to create rotation. With these turbines, the axles are positioned transverse to the wind (as opposed to “in line” with the wind) which has the additional advantage that the vanes do not need to be pointed to the wind. In most cases the axles are positioned vertically (which explains the commonly used acronym VAWT: vertical axle wind turbine), but they may also be positioned horizontally as long as the axle is transverse to the wind (TAWT—transverse axle wind turbine—would thus be a more correct acronym), typically substantially perpendicular to the direction in which the wind blows. These wind turbines however have several disadvantages. Firstly, the overall rate of conversion of kinetic into mechanical energy of these turbines might be improved. Secondly, in particular Darrieus wind turbines are not self-starting. Therefore a small powered motor is required to start off the rotation, and then when it has enough speed the wind passing across the aerofoils starts to generate torque and the rotor is driven around by the wind. An alternative constitution is the use of one or two small Savonius rotors that are mounted on the shaft of the Darrieus turbine to start rotation. These Savonius rotors however slow down the Darrieus turbine when it gets going.
From CN 101737252A a wind turbine according to the preamble is known. This wind turbine is also used to start a Darrieus type of turbine. A disadvantage however is that like other VAWT's, the overall rate of energy conversion is relatively low.
From US2012/0045333 a wind turbine is known wherein the wind receptacle is formed by a sail being blown against a rack, the rack having an adjustable configuration. This way, when the wind is blowing the sail against the rack with too high forces, the receptacle can be reconfigured to have a smaller capacity to convert wind force into motion of the vane simply by letting more wind blow through the various adjacent sails. When the vane travels against the direction of the wind, the sail is blown away from the rack in order to provide a minimum counterforce against the revolving motion of the turbine. Reconfiguration of the rack is then not applied, simply because it has no effect on the capacity of the wind receptacle (i.e. the sail).
U.S. Pat. No. 5,570,997 discloses a wind turbine having a vane provided with a wind receptacle that opens when the vane is traveling in the direction of the wind and closes when the vane is traveling against the direction of the wind. This way, maximum capacity of the wind receptacle is used when the wind force has to be converted into motion of the vane, while the receptacle hardly counteracts this conversion when the vane travels against the direction of the wind. The receptacle opens by action of the wind blowing over the vane and closes under gravity when this wind action decreases.